Candidates and Environment: Dr. Fuerst will conduct anatomical, genetic and morphological experiments related to Aims 1 and 2 at the University of Idaho. Dr. Fuerst's expertise is centered in developmental biology and genetics of retinal development. Fuerst identified the first molecules that regulate retinal mosaics and developed the genetic reagents for use in these experiments. Dr. Borghuis will carry out the electrophysiological and 2-photon fluorescence imaging experiments of Aims 1 and 2 at the University of Louisville. Borghuis discovered that OFF bipolar cells primarily utilize kainate type glutamate receptors and developed the iGluSnFR method to monitor bipolar cell synaptic output. Dr. Borghuis has also developed the computational tools for analyzing imaging data, an established strength of his lab. Research Proposal: Developing neurons readily extend axons and dendrites and make novel synaptic connections, but this ability is severely limited in the adult brain. In our preliminary studies we find evidence that synaptogenesis by retinal bipolar cells is controlled by regulating axons and dendrite growth: while new synapses appear to form, they are restricted to the territory occupied by the bipolar cells' axons and dendrites. We have identified two genes that regulate bipolar cell axonal and dendritic arbor tiling, the Down syndrome cell adhesion molecule (Dscam) and Bcl-2 associated X protein (BAX). Deletion of either gene results in continuous growth and overlap of the axonal and dendritic arbors of OFF bipolar cells. Based on the continuous ability of OFF bipolar cells to make novel anatomical contacts in the wild type retina, and their ability to connect with previously untargeted cells in the Dscam and Bax null retinas, we hypothesize that activating axon and dendrite outgrowth in OFF bipolar cells is sufficient to induce synaptogenesis with targets that would not otherwise be contacted. We test this hypothesis in two specific Aims. Aim 1: we will map normal connectivity patterns of OFF bipolar cells in the developing and aging wild type retina and measure how histological changes during OFF bipolar cell synapse maturation and aging impact functional synaptic connectivity. Aim 2: we will inducibly delete Dscam in adult mice using the Cre:ER system to test if activating dendrite and axon outgrowth in a retina that developed normally is sufficient to induce synaptogenesis. We will determine if the expanding axonal and dendritic arbors that are observed Dscam or Bax null mice establish functional synapses using electrophysiology and 2-photon glutamate imaging. Long-term goals: Our long-term goal is to understand how to activate adult neurons to make novel synapses for use in human therapies. Significance: Understanding why adult mammalian neurons lose the ability to make novel synapses is important because reactivating adult neurons to make new synaptic connections will be critical for regeneration and repair in neurological disease. We will test if activation of dendrite and axon outgrowth is sufficient for OFF bipolar cells to establish synaptic connections with targets they would not otherwise encounter.